U.S. health officials caution governors against easing restrictions, warning that a recent plunge in virus cases ‘may be stalling.’

The director of the Centers for Disease Control and Prevention issued a grave warning on Friday that a recent sharp drop in coronavirus cases across the United States “may be stalling” and “leveling off at a very high number” — a worrisome development that comes as more cases of concerning new variants have been found, and could suggest that a return to normalcy is not yet quite as near as many Americans had hoped.

Even as the vaccination campaign continues, the director, Dr. Rochelle Walensky, also warned governors not to roll back mask mandates or other efforts to contain the virus: “Things are tenuous. Now is not the time to relax restrictions.”

According to a New York Times database, virus cases across the United States appear to be leveling off from the steep decline that began in January, with figures comparable to those reported in late October. Cases have slightly increased week over week in recent days, though severe weather limited testing and reporting in Texas and other states the previous week, and not all states reported complete data on the Presidents Day holiday. The seven-day average of new cases was 77,800 as of Thursday.

While deaths tend to fluctuate more than cases and hospital admissions, Dr. Walensky told reporters during a White House briefing on the pandemic, the most recent seven-day average is slightly higher than the average earlier in the week. The seven-day average of newly reported deaths was 2,165, as of Thursday.

“We at C.D.C. consider this a very concerning shift in the trajectory,” she said, adding, “I want to be clear: cases, hospital admissions and deaths — all remain very high and the recent shift in the pandemic must be taken extremely seriously.”

Dr. Walensky said some of the rise may be attributable to new variants of the coronavirus that spread more efficiently and quickly. The so-called B.1.1.7 variant, which first emerged in Britain, now accounts for approximately 10 percent of all cases in the United States, up from one to four percent a few weeks ago, she said. The U.S. ability to track variants is much less robust than Britain’s.

“I know people are tired; they want to get back to life, to normal,” she said. “But we’re not there yet.”

As cases had declined, some governors around the United States have begun to relax pandemic restrictions. States with Republican governors appeared to be more eager to make rollbacks, though New York, which has a Democrat as governor, has also been easing restrictions on a variety of activities. On Thursday, Gov. Greg Abbott of Texas said he was considering lifting a statewide mask mandate in place since July.

In Mississippi, Gov. Tate Reeves said he was also considering pulling back some restrictions, particularly mask mandates for people who have been fully vaccinated. As of Thursday, just over 12 percent of the state’s population has received at least one shot, and 5.5 percent have received two, according to a Times database.

Dr. Anthony S. Fauci, the government’s top infectious disease expert, echoed Dr. Walensky’s warnings that more rollbacks at state or local levels would be unwise, noting that case levels remained at a “very precarious position.”

“We don’t want to be people always looking at the dark side of things, but you want to be realistic,” he said. “So we have to carefully look at what happens over the next week or so with those numbers before you start making the understandable need to relax on certain restrictions.”

The doctors’ comments came as the Biden administration announced an aggressive push to enlist the help of some of the country’s biggest corporations and business lobbying groups, including the U.S. Chamber of Commerce, the Business Roundtable and organizations representing Asian, Black and Latino executives.

The aim is two-fold: to expand the private sector’s contributions beyond the manufacture of vaccines, tests and treatment, and to encourage businesses to give employees time off and the necessary support to get vaccinated, said Andy Slavitt, a senior Biden health adviser.

Mr. Slavitt ticked off a list of companies and groups that have responded to what he described as the administration’s “call to action”: Ford and The Gap intend to donate more than 100 million masks for free distribution. Uber and Lyft are teaming up with pharmacies to offer free or discounted rides to vaccination sties. Best Buy, Dollar General and Target will give workers paid time off to get a shot. He said the initiatives would be coordinated by the companies themselves and the administration did not have a formal role.

Pro sports leagues, Mr. Slavitt said, are helping set aside more than 100 stadiums and arenas to become mass vaccination sites. A few weeks ago, Mr. Biden announced in a C.B.S. interview that the N.F.L. commissioner had offered him the use of stadiums.

“We’ll ask companies to make similar unique commitments that bring their unique skills and resources to the problem of key keeping Americans safe and ending the pandemic as quickly as possible,” Mr. Slavitt said.